1. Field of the Invention
The present invention relates to a package for hermetically sealing an optical semiconductor equipped with a light transmitting window and for housing an optical semiconductor element in its interior, and to an optical semiconductor module that makes use of this optical semiconductor hermetic sealing package.
2. Description of the Prior Art
The hermetic sealing of an optical semiconductor module is considered very important in optical communications in order to ensure high reliability. This is because the electrodes of an optical semiconductor element will deteriorate under high temperature and humidity, and because moisture that finds its way into the package will condense and diminish the optical characteristics, so the service life of an optical semiconductor element cannot be guaranteed for more than 10 years.
Meanwhile, with an optical semiconductor module, the optical semiconductor element inside the package must be optically linked to an optical fiber on the outside. In view of this, a light transmitting window structure is employed in an optical semiconductor hermetically sealed package in order to optically link the optical semiconductor element on the inside of the package with the optical fiber on the outside while still maintaining the airtightness of the package.
Japanese Patent Application Laid-Open H6-151629 discloses the use of sapphire for a light-transmitting window. Because of its excellent light-transmission and high strength, sapphire is often used in the housings (hermetically sealing packages) of optical semiconductor modules. This sapphire is worked with a drill into a circular shape.
Japanese Patent Application Laid-Open H11-54642 proposes a window structure for an optical semiconductor module, which makes use of a substantially regular hexagonal piece of borosilicate glass for the window. Borosilicate glass is less expensive and has better transmission than sapphire, and it is an isotropic material and does not bring about birefringence. Stress from heat and so on causes elastic strain, which deform the plane of polarization, but it has been found that if the stress is applied uniformly to the glass, as discussed in Japanese Patent Application Laid-Open H11-54642, then the polarization extinction ratio will be reduced (about xe2x88x9240 dB) that any significant problem will not arise. Also, a regular hexagonal shape allows a low-cost dicing process to be used, so another benefit is the reduced cost of the window material. Unfortunately, a drawback is that borosilicate glass is low in strength, and care must be taken when it is to be used under harsher conditions.
The deformation of the plane of polarization is generally expressed by the polarization extinction ratio. In a crossed Nicol prism experimental system, a polarizer on the light-emitting side is rotated 90 degrees relative to that of the light-incoming side, the maximum light intensity is termed Imax and the minimum light intensity Imin, and the polarization extinction ratio is defined as 10xc3x97log10 (Imin/Imax).
Reducing the cost of an optical semiconductor module is of great importance in modern optical communications. This calls for improvement to the window material for a hermetically sealing package that serves as the housing for the optical semiconductor module. However, drill cutting is required to work sapphire crystals into a circular shape as in the above-mentioned Japanese Patent Application Laid-Open H6-151629, and in the course of this working the window material may be scratched or otherwise damaged, which lowers the yield, and this drives up the cost.
As high-density wavelength-multiplexing technology and technology for raising speed have become more sophisticated, it has become imperative that the plane of polarization of the light be maintained and that wavelength uniformity be obtained. To achieve the latter of these two goals it is preferable to form an external resonator structure, such as an optical fiber grating, through a fiber in addition to the hermetically sealing package, but here again it is necessary to maintain the plane of polarization of the light. If this is expressed as the polarization extinction ratio of the intensity level, stringent conditions in which Imax is at least 1000 times Imin (e.g. xe2x88x9230 dB) become necessary. Because of insufficient satisfactory results obtained in Japanese Patent Application Laid-Open H11-54642, the use of the structure proposed therein has still been shunned in fields that require extremely high reliability such as undersea cables.
The present invention was conceived in light of this situation, and it is an object thereof to provide an optical semiconductor hermetic sealing package and an optical semiconductor module that are easy and inexpensive to manufacture, have high reliability, and do not cause the deformation of the plane of polarization.
To achieve the stated object, the present invention provides a hermetically sealing package for an optical semiconductor equipped with a light transmitting window whose light transmitting surface is inclined at least six degrees from the vertical line of the package bottom plate and which is joined to a cylindrical component on the package side wall by the use of a solder brazing material, wherein the window material consists of a light-transmitting ceramic plate in which a metallized portion is formed around the periphery, leaving a circular light transmitting portion in the center of the plate. The present invention further provides a hermetically sealing package for an optical semiconductor in which the plate is made of a sintered body of an oxide containing aluminum (such as alumina or spinel). Methods for manufacturing a light-transmitting spinel sintered body have been disclosed in Japanese Patents 2620287 and 2620288.
The present invention provides an optical semiconductor module having an optical semiconductor element inside the above-mentioned optical semiconductor hermetic sealing package of the present invention, and an optical fiber outside the same package.
Since a light-transmitting ceramic is easier to work than sapphire, a disk can be easily produced by drilling, which helps lower the cost. Also, this material can be easily diced into a regular hexagonal shape, just as with borosilicate glass.
A regular hexagonal shape affords the highest yield when low-cost dicing is performed. The yield is even higher than with conventional drilling because the tip of a dicing blade is smaller than a drill tip. An adhesive wafer sheet can be used with dicing, so there is no chip scattering, and window materials can be produced at a higher yield, so the cost is lower.
With monoaxial crystals such as sapphire, birefringence does not occur if the optical axis is aligned with the C-axis of the crystals. If the incident light is linearly polarized light, the C-axis will lie within the plane of polarization, and no birefringence will occur. To ensure a polarization extinction ratio of xe2x88x9230 dB, which is what is required in the field of optical communications, the plane of polarization must be aligned with the C-axis plane. A large polarization extinction ratio is a source of loss and noise. If the window material is made from a sintered body of a light-transmitting ceramic, however, it will be in a polycrystalline state (a collection of minute crystals) and will be optically isotropic, and deformation of the plane of polarization when light passes through can be eliminated regardless of the orientation of the window material. Accordingly, an optical semiconductor hermetic sealing package can be easily produced without any positioning, which affords a lower cost.
The optical semiconductor hermetic sealing package of the present invention can be used to produce an optical semiconductor module that is low in cost and high in reliability, and with which there is no deformation of the plane of polarization.